May Gardening; Dirt is what politicians sweep under the carpet. Soil is what plants grow in

Questions We’re Often Asked: Soil vs. Dirt

“It’s like trying to become a gardener without touching the dirt,” Christopher Perillo, a science teacher in Kenosha, Wisconsin, was quoted as saying in New Scientist magazine. Perillo may be a science teacher but he would be a dubious gardener with that attitude. Soil is not dirt. Dirt is what politicians sweep under the carpet. Soil is what plants grow in and, ultimately, what feeds and shelters us – even politicians and science teachers.

Soil is the upper layer of earth that clothes our planet. It may be but inches or, rarely, many feet deep. Soil forms as rock weathers and dissolves, with the help of soil organisms, creating particles that bind with decaying biomass and living microbes to form larger aggregates, explained Steve Banwartin in an article for the journal Nature. In a simplified classification, the resulting soils are known as sand, silt, or clay. Scientists who study soil, however, have thousands of names for those found in different locations around the world.

Indeed, its underlying soil may determine whether a country is rich or poor. Chad is dirt poor because its soil is poor. Germany is relatively rich because its soil is rich. That’s the provocative conclusion, noted the journal Science, of a new study by researchers at the University of Basel in Switzerland, which appeared online in PloS One.

Soil is not, however, merely particles of rock. These, clothed with water, also contain a surprising volume of air. There are also quantities of organic matter – plant and animal material that was once living but is being broken down by countless numbers of microbes. A single gram of topsoil may contain a billion individual microbial cells encompassing tens of thousands to a million different species, notes Charles Petit in Science News. Only a fraction of those species have been cultured in labs or even named, he adds.

Soil forms slowly, probably less than an inch a century in the northern U.S. and southern Canada. Yet this invaluable resource is treated as, well, dirt. Paved and roofed over by urban settlements, compacted by machinery, scraped aside without a care. It may be polluted by poisons, washed away in storms, and blown away during droughts. Soil losses in some locations around the world are in excess of 50 tonnes per hectare in a single year: up to 100 times faster than the rate of soil formation, notes Steve Banwart.

And in surely the most ironical of extremes, it may be stripped from fields, bagged and sold back to city gardeners to restore their properties or even to fill planters built because the ground below them can no longer support plant life.

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